SnoN and c-Ski are nuclear proto-oncogenic proteins that have the unusual property of being able to affect cellular proliferation and also initiate terminal skeletal muscle differentiation, in which cells withdraw from the cell cycle. It is thought that the Ski family proteins achieve their transforming ability by antagonizing the growth inhibitory functions of TGF-b signaling. SnoN is less potent than c-Ski in inducing transformation and myogenesis, and little is known about how the weak property is controlled. Thus, the paradoxical effect of SnoN and Ski on cell transformation and differentiation has not been completely understood. In this application, we will focus our study on how SnoN is regulated by SUMO modification. Our hypothesis is that SUMOylation controls the biological activities of SnoN in transformation and cell differentiation. Two specific aims are proposed: 1. Investigate the molecular interactions and requirements of SnoN SUMOylation. We will examine biochemically the SUMO modification of SnoN. Potential SUMO E3 ligases will be tested for their role in SnoN SUMOylation, and their interactions with SnoN will be analyzed. Subcellular location where SnoN SUMOylation occurs will also be investigated. 2. Elucidate the physiological functions of SnoN SUMOylation in cell proliferation and myogenic differentiation. SUMOylation of SnoN proteins under various conditions for cell proliferation or differentiation will be analyzed to determine whether there is a correlation between the SUMOylation status of SnoN and its activity in cell regulation. Gain-of-function or loss-of-function approaches will also be used to elucidate the functions of SnoN SUMOylation in cell transformation and myo-differentiation. Molecular basis for the altered functions of SnoN will be determined. The current proposal will open up a new area of research on how the activities of Ski family proteins are regulated by post-translational modification. Particularly, the study on the role of SUMO modification in SnoN protein function regulation will contribute to our understanding of the biology of SnoN/c-Ski in the coordinated regulation of cell proliferation and differentiation. Finally, the results may also provide a foundation for the rational design of novel therapeutic approaches for prevention and treatment of cancer and musculoskeletal diseases. [unreadable] [unreadable]